Are you able to post any error messages you get in your Event Viewer(Right click My Computer go to Manage, then Event Viewer, under application and system..) Post any errors here.
Also if it is locking up, if you know when it is going to lockup (After a set amount of time, certain program/file that freezes it) Try in safemode to recreate this (To go into safemode, shutdown or restart the computer, while booting up, tap F8. Safe Mode with Networking if you need to use the internet)
If this is after a recent change to the computer, try a system restore. Start>All Program>Accessories>System Tools>System Restore
Possibly test your RAM with Memtest if you can, and your Hard Drive, Hiren's Boot CD is a very helpful tool in all area's..
Other then that, that's a few things that come to mind, run chkdsk and SFC through Command Prompt (Go to Run, or hold the windows key and hit R) type CMD. In the black command prompt type cd \, then type chkdsk /f /r hit yes, reboot computer.
Hopefully it does the checkdisk, if that doesnt work try going back to command prompt, type cd \ then sfc /scannow you will need your windows installation CD for this to work, but more then likely this wont fix it, this is more for missing/corrupt windows files but worth a shot if nothing has worked
